Optical pick-up for use with an opto-magnetic signal

ABSTRACT

An optomagnetic information recording and reproducing apparatus includes an optomagnetic optical pick-up for recording or reproducing information on or from an optomagnetic information recording disc. An optomagnetic optical pick-up includes a first diffraction grating for receiving light reflected from the disc and separating the light into a first transmitting light which transmits through the first diffraction grating beam and a first diffracted light beam which is diffracted by the first diffraction grating. A second diffraction grating is disposed to receive the first transmitging and diffracted light beams and discharges a second transmitting light beam and a second diffracted light beam. A tracking error detector is provided to receive the second transmitting light beam and a focusing error detector is disposed to receive the second diffracted light beam. An optomagnetic signal is obtained as a difference between outputs from the tracking and focusing error detectors.

This is a division of application Ser. No. 07/294,466 filed Jan. 6,1989, now U.S. Pat. No. 5,115,423.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to an optical information recording andreproducing apparatus, and, in particular, to an optomagnetic (ormagnetooptic) information recording and reproducing apparatus foroptically recording or reproducing information on or from anoptomagnetic recording medium, such as an optomagnetic disc, byutilizing the direction of polarization.

2. Description of the Prior Art

FIG. 9 illustrates a typical prior art optical pick-up capable for usein recording, reproducing or erasing information on or from anoptomagnetic disc (not shown). The optical pick-up includes an objectivelens 1 located facing an optomagnetic disc, an illumination opticalsystem 2, a servo optical system 3 and an optomagnetic detection opticalsystem 4, which are all mounted on a substrate 5. In the illuminationoptical system 2, laser light emitted from a semiconductor laser 6 iscollimated by a coupling lens 7 and the thus collimated light is passedthrough beams-shaping splitters 8 and 9 to thereby define a light beamcircular in shape. Then, this light beam is incident upon a polarizationbeam splitter 10 which serves to separate the incident light beam from areflected light beam and which is provided with a polarizing surface 10afor allowing p waves to be transmitted at 100% and reflecting s waves at2/3, so that only the s wave components are reflected by the polarizingsurface 10a and the reflected light beam then passes through theobjective lens 1 to be directed toward an optomagnetic disc (not shown).

When the light is reflected by the optomagnetic disc, it is rotatedeither in the positive or negative direction over a predetermined angleof polarization depending on the recording condition on the optomagneticdisc. Thus, the reflected light beam includes p waves and is directedtoward beam splitters 10 and 11. Thus, the s waves reflected by thepolarizing beam splitter 11 for separating a servo detecting system andan optomagnetic detection optical system travel through a detection lens12 of the servo detection system 3 and a knife edge prism 13 and arereceived by a tracking light detector 14 and a focusing light detector15. A tracking detection is carried out by a so-called push-pull method.The s waves transmitting through the polarization beam splitters 10 and11 and the p waves produced by reflection have their planes ofpolarization rotated over 45° when passing through a half wavelengthplate 16 located next to the polarization beam splitter 11 in theoptomagnetic detection optical system 4. Furthermore, p and s wavecomponents are separated by a Wollaston prism 17 and then an image isformed on a light detector 19 by a detection lens 18 to thereby detectthe direction of polarization, whereby the presence of absence of arecorded signal is detected.

However, in such a prior art structure, since tracking signal detection,focusing signal detection and optomagnetic signal detection are carriedout by respective, separate optical systems, there must be provided agreat number of optical components. In this case, there must be providedthe half wavelength plate 16 and the Wollaston prism 17 as opticalcomponents for detecting the direction of polarization of reflectedlight in the optomagnetic detection optical system 4, and, thus, theoptical system is rather complicated in structure. In particular,positioning must be carried out for the polarizing beam splitter 10,half wavelength plate 16 and Wollaston prism 17 individually. As aresult, the number of steps in assembly increases and the stabilitytends to be impaired. Moreover, since there must be provided such agreat number of optical components, the entire apparatus is rather heavyin weight and the access speed is rather slow.

In order to provide a high-speed access time in an optomagnetic discapparatus, it is imperative to make an optical pick-up compact in sizeand light in weight. In this respect, there has previously been proposedan optical pick-up utilizing a high density diffraction grating as shownin FIG. 10, which was the subject of a Japanese Patent Application whichhas been assigned to the assignees of this application and thus herebyincorporated by reference. In the structure of FIG. 10, similarly withthe case of FIG. 9, laser light emitted from a semiconductor laser 21travels through a coupling lens 22, beam shaping splitters 23 and 24, apolarizing surface 25a of a polarization beam splitter 25 and anobjective lens 26 and is focused onto an optomagnetic disc (not shown).The light reflected from the optomagnetic disc again travels through theobjective lens 26 and the polarizing beam splitter 25 and then isseparated from the incoming light beam to be directed toward a lens 27.Thereafter, the light enters a high density diffraction grating 28 whichis inclined at a predetermined angle, where the incoming light isseparated into transmitting light 29 of 0th order light and diffractedlight 30 of 1st order light. The transmitting light 29 is received by a4-division light-receiving device 31 for use in detecting a focusingsignal; on the other hand, the diffracted light 30 is received by a2-division light-receiving device 32 for use in detecting a trackingsignal.

The high density diffraction grating 28 typically has such apolarization dependency characteristic as shown in FIG. 11. Thus,detection of an optomagnetic signal recorded on an optomagnetic disc iscarried out as a difference between the outputs from the light-receivingdevices 31 and 34. That is, with an angle of polarization denoted byalpha, the use rate of the 1st order light is approximately equal tosin² alpha and the use rate of the 0th order light is approximatelyequal to cos² alpha, so that a difference between the two is sin²alpha-cos² alpha. Detection of a focusing signal is carried out by anoutput from the 4-division light-receiving device 31 utilizingastigmatism which is produced by the lens 27 and the high densitydiffraction grating 28. Detection of a tracking signal is carried out bythe 2-division light-receiving device 32 according to a push-pull methodutilizing the 1st order light (diffracted light 30).

With this optical pick-up structure, the number of required componentsmay be reduced and the overall structure may be made compact in size andlight in weight as compared with the optical pick-up structure shown inFIG. 9. However, since use is made of the high density diffractiongrating 28, the diffraction angle of the 1st order light would deviatesignificantly due to fluctuations in the wavelength of laser light fromthe semiconductor laser 21. For example, if a constant n of the highdensity diffraction grating 28 is equal to 1.5, when the wavelengthabruptly changes by 2 nm, a light spot will be shifted over 0.15 mm at apoint 30 mm away from the grating. Besides, since the twolight-receiving devices 31 and 34 are spaced apart from each other overa relatively long distance, there is a difficulty in assembly and alsoin adjustments. In particular, there is a possibility that the trackingsignal detecting light-receiving device 32 is located in a directionvertical to the plane of the drawing, in which case difficulty isincreased even more.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided an improvedoptomagnetic information recording and reproducing apparatus in whichlight from a light source is irradiated onto an optomagnetic recordingmedium to thereby carry out recording or reproducing of information. Theapparatus includes a light separating means having a first diffractiongrating, into which reflected light from the optomagnetic recordingmedium is incident, and a second diffraction grating. The 0th and 1storder light emitted as separated from the first diffraction grating areboth incident upon the second diffraction grating from which the 0th and1st order light are again emitted as 0th and 1st order light asseparated. The apparatus also includes a tracking signal detecting lightdetector which receives either one of the 0th and 1st order lightemitted from the second diffraction grating and a focusing signaldetecting light detector which receives the other of the 0th and 1storder light emitted from the second diffraction grating. Then, anoptomagnetic signal is obtained as a difference between the outputs fromthese light detectors.

It is therefore a primary object of the present invention to obviate thedisadvantages of the prior art as described above and to provide animproved optomagnetic information recording and reproducing apparatus.

Another object of the present .invention is to provide an improvedoptomanetic information recording and reproducing apparatus compact insize and light in weight.

A further object of the present invention is to provide an improvedoptomagnetic information recording and reproducing apparatus fast andreliable in operation.

A still further object of the present invention is to provide animproved optomagnetic information recording and reproducing apparatuswhich is least affected by fluctuations in the wavelength of lightemitted from a light source for use in recording or reproducinginformation.

A still further object of the present invention is to provide animproved optomagnetic information recording and reproducing apparatuswhich requires a minimum number of components and thus which is easy tomanufacture and carry out adjustments.

A still further object of the present invention is to provide animproved optical information recording and reproducing apparatusutilzing the direction of polarization in recording and reproducinginformation to and from an optical disc.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration showing an optomagnetic informationrecording and reproducing apparatus constructed in accordance with oneembodiment of the present invention;

FIG. 2 is a schematic illustration showing a double diffraction gratingprovided in the structure shown in FIG. 1;

FIG. 3 is an illustration which is useful for explaining the function ofthe double diffraction grating shown in FIG. 2;

FIG. 4 is a graph showing a relationship between the polarization angleand the differential coefficient of the light use rate of a differencebetween 0² and 1² order light for inputting polarization angle;

FIG. 5 is an illustration showing a modified example of the doublediffraction grating;

FIG. 6 is an illustration which is useful for explaining the function ofthe double diffraction grating shown in FIG. 5;

FIG. 7 is a schematic illustration showing another modified example ofthe double diffraction grating;

FIG. 8 is an optomagnetic information recording and reproducingapparatus constructed in accordance with another embodiment of thepresent invention;

FIG. 9 is a schematic illustration showing in perspective view a typicalprior art optical pick-up for use in an optomagnetic informationrecording and reproducing apparatus;

FIG. 10 is a schematic illustration showing another prior art opticalpick-up for use in an optomagnetic information recording and reproducingapparatus;

FIG. 11 is a graph showing a relationship between the polarization angleof a diffraction grating and the light utilization efficiency;

FIG. 12 is a schematic illustration showing in perspective view anoptical information reading apparatus constructed in accordance with oneembodiment of the present invention;

FIG. 13 is an illustration showing a 4-division light-receiving devicewith an associated circuit structure used in the apparatus of FIG. 12;

FIG. 14 is a schematic illustration showing an optical informationreading apparatus constructed in accordance with another embodiment ofthe present invention;

FIG. 15 is a schematic illustration showing a modified structure inwhich an optical element and a second polarization light beam splitterare combined in one unit;

FIG. 16 is a schematic illustration showing a typical prior art opticalinformation recording and reproducing apparatus;

FIG. 17 is a schematic illustration showing an optomagnetic opticalpick-up for use in an optomagnetic information recording and reproducingapparatus constructed in accordance with one embodiment of the presentinvention;

FIG. 18 is a schematic illustration showing the structure of thehologram used in the structure shown in FIG. 17;

FIG. 19 is a schematic illustration showing the structure of the2-division light-receiving device used in the structure shown in FIG.17;

FIG. 20 is a schematic illustration showing the structure of the4-division light-receiving device used in the structure shown in FIG.17;

FIG. 21 is a schematic illustration showing another optomagnetic opticalpick-up for use in an optomagnetic information recording and reproducingapparatus constructed in accordance with another embodiment of thepresent invention;

FIGS. 22 and 23 are illustrations showing the structures of therespective holograms used in the structure of FIG. 21;

FIG. 24 is a schematic illustration showing the overall structure of atypical prior art optomagnetic information recording an reproducingapparatus;

FIGS. 25 through 27 are schematic illustrations showing modifications ofthe optomagnetic optical pick-up;

FIG. 28 is a schematic illustration showing the structure of a hologramused in the structure shown in FIG. 10 or 11;

FIG. 29 is a schematic illustration showing the overall structure of anoptomagnetic optical pick-up constructed in accordance with oneembodiment of the present invention;

FIGS. 30a through 30c are schematic illustrations showing the structuresof the respective light-receiving devices used in the structure shown inFIG. 29;

FIGS. 31a through 31c are schematic illustrations showing the structuresof the respective hologram diffraction gratings used in the structureshown in FIG. 29;

FIG. 32 is a schematic illustration showing the overall structure of anoptomagnetic optical pick-up constructed in accordance with anotherembodiment of the present invention;

FIG. 33 is a schematic illustration showing the overall structure of atypical prior art optomagnetic optical pick-up for use in anoptomagnetic information recording and reproducing apparatus; and

FIG. 34 is a schematic illustration showing the overall structure ofanother typical prior art optomagnetic optical pick-up.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIGS. 1 through 4, an optomagnetic informationrecording and reproducing apparatus constructed in accordance with oneembodiment of the present invention will be described. The presentapparatus has a basic structure which is similar in many respects tothat of an optical pick-up shown in FIG. 10 so that like elements areindicated by like numerals. In the first place, in place of the highdensity diffraction grating 28 of FIG. 10, provision is made of a doublediffraction grating 40 as a light separating means. This doublediffraction grating 40 is integrated in structure and of the lighttransmitting type, and it includes a substrate, a first diffractiongrating 41 provided on one surface of the substrate (light incident sidefacing a lens 27) and a second diffraction grating 42 provided on theopposite surface of the substrate (light emitting side). The doublediffraction grating 40 is disposed as inclined at a predetermined anglein the optical path (optical axis of the objective lens 26) for thereflected light 39 from an optomagnetic disc as an optomagneticinformation recording medium.

At the light-emitting side of the double diffraction grating 40 aredisposed a 2-division light-receiving device 44 as a tracking signaldetecting light detector for receiving, for example, the diffractedlight 43 of 1st order light and a 4-division light-receiving device 46as a focusing signal detecting light detector for receiving thetransmitted light 45 of 0th order light. With this structure, a focusing(error) signal is detected by an astigmatic method using the 4-divisionlight-receiving device 46 for receiving the transmitted light 45 fromthe double diffraction grating 40, and a tracking (error) signal isdetected by a push-pull method using the 2-division light-receivingdevice 44 which receives the diffracted light 43. And, an optomagneticsignal from an optomagnetic disc is obtained by taking a differencebetween the outputs from the light-receiving devices 44 and 46.

The provision of the double diffraction grating 40 is important in thepresent embodiment, and the direction of diffraction in the doublediffraction grating 40 will be explained with reference to FIG. 3.Incident light Ei of the reflected light 39 incident upon the doublediffraction grating 40 through the lens 27 is diffracted by the firstdiffraction grating 41 having a grating constant K₁ under a phasematching condition in the Z direction, so that it is separated into thetransmitting 0th order light and the diffracted 1st order light. The 1storder light diffracted by the first diffraction grating 41 is againdiffracted by the second diffraction grating 42 having a gratingconstant K₂, and there is emitted diffracted light Ed having a directionin vector representation in FIG. 3. Since the diffracted light Ed hasbeen diffracted twice, i.e., once by the diffraction grating 41 and forthe second time by the diffraction grating 42, it may be referred to as1st² order light (=diffracted light 43) as compared with simple 1storder light. Similarly, since the incident light Ei=transmitted light 45passes through both of the diffraction gratings 41 and 42, it may bereferred to as 0th² order light.

Such 0th² and 1st² order light may be separated when emitted at apredetermined separation angle θ by appropriately setting the gratingconstants K₁ and K₂ of the diffraction gratings 41 and 42. In this case,constants K₁ and K₂ of the diffraction gratings 41 and 42 of the doublediffraction grating 40 may be so set to have a characteristic similar tothe light utilization efficiency shown in FIG. 11. Thus, the lightutilization efficiencies of the 1st² and 0th² order light becomeapproximately equal to sin⁴ alpha and cos alpha, respectively. Adifference between the two may thus be expressed in the followingmanner. ##EQU1##

This indicates the fact that, even in the present embodiment using thedouble diffraction grating 40, there is obtained a result similar to thedifference in light utilization efficiency between the 0th and 1st orderlight in the case of using the high density diffraction grating 28having a single diffraction grating structure shown in FIG. 10. As shownin FIG. 4, a differential coefficient of light utilization efficiency ofa difference between the 0th² and 1st² order light for an inputpolarization plane by the double diffraction grating 40 may be expressedin the following manner. ##EQU2## In this manner, in accordance with thepresent embodiment, there is simply provided the double diffractiongrating 40 in place of the high density diffraction grating 28 of FIG.10; however, there are following advantages as compared with thestructure shown in FIG. 10. In the first place, even if fluctuationsoccur in the laser light emitted from the semiconductor laser 21, thereis produced almost no shift in the 1st² order light (diffracted light43) coming out of the double diffraction grating 40. For example, if thegrating constant n=1.5, the amount of a shift of a light spot at aposition 30 mm away from the double diffraction grating 40 for 2 nm offluctuation in wavelength is very small and in the order of 2 micronmeters. Thus, it may be understood that the present optical pick-up isleast affected by fluctuations in wavelength. Second, in the presentembodiment, the light-receiving device 44 for detecting a trackingsignal and the light-receiving device 46 for detecting a focusing signalmay be arranged extremely closely to each other. In addition, thedistance between these light-receiving devices 44 and 46 may be setvariably appropriately by appropriately setting and adjusting therespective grating constants K₁ and K₂. Thus, in assembly, it isrequired to adjust only once for one of the detectors (light-receivingdevices 44 and 46) to the other. Moreover, there is no possibility thatthe light-receiving device 44 is located in the vertical direction withrespect to the plane of the drawing and it may be arranged on the planeof the drawing together with the light-receiving device 46, so that itsassembly is extremely simple. Furthermore, similarly with the structureshown in FIG. 9, there may be provided an optical pick-up fewer in thenumber of parts, light in weight, compact in size and fast in accessoperation.

In the above-described embodiment, it is so structured that detection ofa focusing signal is carried out using the transmitted light 45 (=0th²order light) coming out of the double diffraction grating 40 anddetection of a tracking signal is carried out using the diffracted light43 (=1st² order light). However, it may also be so structured to detecta focusing signal using the diffracted light 43 and a tracking signalusing the diffracted light 45. In this case, the diffracted light 43from the double diffraction grating 40 is detected by the 4-divisionlight-receiving device 46 for detecting a focusing signal and thetransmitted light 45 is received by the 2-division light-receivingdevice 44 for detecting a tracking signal.

As shown in FIG. 5, the double diffraction grating 40 may be sostructured that it includes a substrate which is generally wedge-shapedand the diffraction gratings 41 and 42 provided on the opposite surfacesof the substrate have the same grating constants K₁ and K₂. FIG. 6illustrates a vector representation indicating the direction ofdiffraction in the case of the double diffraction grating 40 shown inFIG. 5. In this double diffraction grating 40, since the diffractiongratings 41 and 42 have the same grating constants, it can bemanufactured with ease.

In the above-described embodiment, the grating vectors of the respectivediffraction gratings 41 and 42 are made different relative to eachother. In an alternative structure, the grating constants of therespective diffraction gratings 41 and 42 may be made equal whilemaintaining the substrate as a parallel plate. FIG. 7 illustrates such amodified embodiment, in which separation between the 0th² and 1st² orderlight is attained by the thickness of the substrate of the doublediffraction grating 40. With this structure, there will be nofluctuations in the output angle of the 1st² order light due tofluctuations in the wavelength of the laser light. Furthermore, thedouble diffraction grating 40 may be of the reflecting type instead ofthe transmitting type as in the present embodiment. And, as an opticalsystem as a whole, the light source side and the detecting system sidemay be replaced with the polarization beam splitter 25 as a boundary inFIG. 1.

FIG. 8 schematically illustrates an optical pick-up for use in anoptomagnetic information recording and reproducing apparatus constructedin accordance with another embodiment of the present invention. Asshown, this embodiment is constructed by omitting the polarization beamsplitter 25. The laser light emitted from the semiconductor laser 21disposed in the vicinity of the 4-division light-receiving device 46passes through the coupling lens 22 and travels through the diffractiongratings 42 and 41 of the double diffraction grating 40 before itreaches an optomagnetic disc (not shown). That is, the 0th² order lightpassing through the double diffraction grating 40 is irradiated to theoptomagnentic disc. And, the light reflected from the optomagnetic discis again incident upon the double diffraction grating 40 and the 1st²order light coming out of the double diffraction grating 40 again passesthrough the coupling lens 22 to be focused onto the 4-divisionlight-receiving device 46 with astigmatism so that a focusing signal isdetected. The astigmatism may be appropriately set in consideration ofthe grating constants of the diffraction gratings 41 and 42 of thedouble diffraction grating 40 and the thickness thereof.

On the other hand, the 0th order light - 1st order light or 1st orderlight - 0th order light 47 obtained by passing the light through thediffraction gratings 41 and 42 of the double diffraction grating 40 insuccession is focused onto the 2-division light0-receiving device 44 bya lens 48, whereby a tracking signal is detected by the push-pullmethod. In this case, since the 0th order light - 1st order light or the1st order light - 0th order light 47 is influenced by fluctuations inwavelength, it is preferable to use a 2-division light-receiving devicewhose division line is parallel to the direction of fluctuation as the2-division light-receiving device 44. Alternatively, the gratingconstant may be set to be small such that the separation angle betweenthe 0th² order light and the 1st² order light is small and thecollimating and shaping lens 22 may also serve as the lens 48 fordetecting a tracking signal. In this case, however, the ability todetect an optomagnetic signal somewhat deteriorates. In the abovedescription, the generation of astigmatism for detecting a focusingsignal has been explained to be influenced by the grating constant andthe thickness. Alternatively, use may be made of diffraction gratingshaving modulated grating constants, such as varying grating pitch in astepwise fashion, as the diffraction gratings 41 and 42.

FIG. 16 illustrates another typical prior art optical informationrecording and reproducing apparatus. The light emitted from asemiconductor laser 101 passes through a coupling lens 102 to becollimated and then passes through a beam shaping splitter 103 wherebythe plane of polarization is changed from an ellipse to a circle. Thelight which has been shaped to be circular then passes through a firstpolarization beam splitter 104, which allows 100% of the p-polarizedlight to transmit and causes 66% of the s-polarized light to reflect(this characteristic also holds true for a later-described secondpolarization beam splitter) so that only the s-polarized light travelsthrough a polarization prism 105 and an objective lens 106 to be focusedonto an optical disc 107. Then, the light is reflected in the form ofp-polarized waves depending on the direction of magnetization of theoptical disc as an optical information recording medium.

The light reflected from the optical disc 107 then passes through thefirst polarization beam splitter 104 and then it is split into two beamsby a second polarization beam splitter 108, one of which passestherethrough to be directed toward an optomagnetic detection opticalsystem 109 and the other of which is deflected to be directed toward aservo detection system 110. In the optomagnetic detection optical system109, the reflected light containing p-polarized waves transmittingthrough the first polarization beam splitter 104 is rotated over 45° bya half wavelength plate 111 and then separated between normal light andabnormal light by a Wollaston prism 112, which are then led toward alight-receiving device 114 having a 2-division light-receiving surfacethrough a detection lens 113, so that an optomagnetic signal recorded onthe optical disc 107 can be detected. With this, the amount of each ofthe normal and abnormal light can be determined and it can be detectedwhether or not the direction of polarization has been changed or not.

In the servo optical system 110, the light of s-polarized wavesreflected by the second polarization beam splitter 108 is focused by thefocusing lens 115 and then divided into two light beams by a knife edgeprism 116. And, the light reflected by the knife edge prism 116 is ledto a tracking error detecting light-receiving device 117 whereby atracking error signal is detected to carry out a tracking servo controloperation. On the other hand, the light which has been separated andwhich has advanced straight is directed toward a focusing errordetecting light-receiving device 118, whereby a focusing error signal isdetected to carry out a focusing servo control operation.

In the case of the above-described prior art structure, since the numberof parts is large, its assembly and adjustment take time and the overallstructure tends to be bulky, which tend to push up the cost.

This aspect of the present invention has been particularly directed tosolve the problems set forth immediately above and to provide animproved optical information recording and reproducing apparatus fewerin the number of parts, compact in size, light in weight and low atcost. In accordance with this aspect of the present invention, there isprovided an improved optical information recording and reproducingapparatus which comprises an optical element which is disposed in anoptical path for light reflected by a polarization beam splitter andwhich is formed with a first linear diffraction grating differing ingrating spacing at its light input surface and with a second lineardiffraction grating differing in grating spacing in a directionperpendicular to the direction in which the first diffraction grating isformed at its light output surface to which the light passing throughthe light inlet surface and reflected by a reflecting surface isdirected. A 4-division light-receiving device is provided in an opticalpath for the light output from the light output surface. Thus, the lightoutput from the optical element is directed toward the 4-divisionlight-receiving device to carry out a focusing servo control operation.With this structure, the number of parts required in the servo opticalsystem can be reduced so that there can be provided an apparatus lightin weight, compact in size and low at cost.

In addition, a 2-division light-receiving device may be provided incontact with the reflecting surface. In this case, a tracking servocontrol operation can also be carried out.

Referring now to FIGS. 12 and 13, an optical information recording andreproducing apparatus constructed in accordance with one embodiment ofthe present invention will be described in detail below. It is to benoted that like numerals used in FIG. 16 will be used for like elements.

An optical element 119 is in the shape of a triangular prism and it isformed with a light input surface 120 into which the light reflectedfrom an optical disc 107 is input, with a reflecting surface 121 forreflecting the light passing through the light input surface 120 andwith a light output surface 122 through which the light reflected by thereflecting surface 121 is output. The light input surface 120 is formedwith a first linear diffraction grating 123 in parallel with X directionsuch that the grating spacing T is coarse at the center and becomesgradually dense toward the ends. The light output surface 122 is formedwith a second linear diffraction grating 124 extending in Z directionperpendicular to the direction along which the first diffraction grating123 is formed and similar in structure to the first diffraction grating123.

The optical element 119 is located in an optical path for the lightwhich is reflected by the second polarization beam splitter 108 (seeFIG. 16) for the reflected light from the optical disc 107. In addition,a 4-division light-receiving device 125 for carrying out a focusingservo control operation is provided in an optical path for the lightoutput from the light output surface 122 of the optical element 119.

A method for carrying out a focusing servo control operation using theoptical element 119 will be described below. The light which has beenreflected by the second polarization beam splitter 108 and which hasbeen input into the light input surface 120 of the optical element 119is focused in Y direction by the first diffraction grating 123 so thatits beam shape is changed from a circular shape into an ellipseelongated in X direction. And, the light thus deformed is reflected bythe reflecting surface 121 and then output through the light outputsurface 122, whereby the light thus modified is focused by the seconddiffraction grating 124 in X direction so that its beam shape is changedinto an elliptical shape elongated in Z direction.

Thus, the light output through the light output surface 122 of theoptical element 119 is a light beam having astigmatism. Thus, with the4-division light-receiving device 125 disposed at a position P in theoptical path where a beam shape becomes circular, a focusing servocontrol operation can be carried out according to a well-knownastigmatic method.

Now, FIG. 14 schematically illustrates an optical element 119constructed in accordance with another embodiment of this aspect of thepresent invention. The present optical element 119 is so structured toallow to carry out not only a focusing servo control operation, but alsoa tracking servo control operation. Thus, a description regarding afocusing servo control operation of this optical element 119 will beomitted and only its tracking servo control operation will be described.It is to be noted that like numerals indicate like elements in theprevious embodiment.

The present optical element 119 includes a 2-division light-receivingdevice 126 directly attached to its light reflecting surface 121. Thereflecting surface 121 is set at a predetermined angle such that aportion of the light transmitting through the light input surface 120transmits therethrough to enter the 2-division light-receiving device126 and the remaining portion of the light transmitting through thelight input surface 120 is reflected to be directed toward the lightoutput surface 122. With this structure, a light beam which has enteredthe light input surface 120 of the optical element 119 and then whichhas been deformed to become elongated in X direction by the firstdiffraction grating 123 transmits through the reflecting surface 121 tobe led into the 2-division light-receiving device 126 so that a trackingerror signal can be detected by the well-known push-pull method tothereby carry out a tracking servo control operation.

As described above with reference to the first and second embodiments,in a servo optical system which carries out focusing and tracking servocontrol operations, the optical element 119 may replace such componentsas focusing lens 115 and knife edge prism 116. The first and seconddiffraction gratings 123 and 124 differ in structure in that theirdirections of gratings are perpendicular to each other, and there arenot any other constraints as to the differences between these twogratings. If the diffraction gratings 123 and 124 are set to have thesame grating spacing T, they may be formed from one kind of a mold,which contributes to lower the manufacturing cost. As an alternativestructure, the second beam splitter 108 may be integrated with theoptical element 119 as shown in FIG. 15, in which case, the requiredinstallation space may be reduced significantly and the apparatus can bemade smaller in size and lighter in weight.

A further aspect of the present invention will now be described below.This aspect of the present invention relates to an optical pick-up foruse in an optical information recording and reproducing apparatusutilizing the light emitted from a semiconductor laser. In the firstplace, FIG. 24 illustrates a typical prior art optomangetic opticalpick-up for use in an optomagnetic information recording and reproducingapparatus. The light emitted from a semiconductor laser 201 iscollimated by a collimator lens 202 and the thus collimated light ischanged from an elliptical cross section to a circular cross section bya beam shaping prism 203. The light whose cross section has been changedto a circular cross section passes through a first beam splitter 204, isreflected by a total reflection mirror 205, passes through an objectivelens 206 to be focused onto an optomagnetic disc 207 as an opticalinformation recording medium, where information may be recorded. And,the light reflected from the optomagnetic disc 207 is reflected by thefirst beam splitter 204 and then divided into two light beams by asecond beam splitter 208, where one of the divided light beams transmitstherethrough to be directed toward a servo optical system 209 and theother of the divided light beams is reflected to be directed toward anoptomagnetic detection system 210.

In the servo optical system 209, the light beam passes through a pair oflenses 211 and to be focused onto a light-receiving device 212, wherebya focusing error signal and a tracking error signal are detected tothereby carry out focusing and tracking servo control operations. In theoptomagnetic optical system 210, the light beam passes through a halfwavelength plate 213 and a focusing lens 214 and is divided into twobeams by a polarization beam splitter 215, which are then focused ontorespective light-receiving devices 216 and 217 to thereby produce anoptomagnetic signal from which the information recorded on the opticaldisc 207 is reproduced.

In such an apparatus, various proposals have been made to endeavor toreduce the number of parts. For example, FIGS. 25 through 27 illustrateseveral such examples which are so structured to reduce the number ofparts particularly in the optomagnetic optical system 210. In FIG. 25,instead of the above-described polarization beam splitter 215, provisionis made of a Wollaston prism 218 which divides a light beam into twolight beams which are then focused by a lens 219 onto a 2-divisionlight-receiving device 220 to thereby detect an optomagnetic signal.However, it cannot be said that the number of parts is reduced in such astructure. In FIG. 26, instead of the polarization beam splitter 215,provision is made of a hologram 222 (see FIG. 28) provided with straightgrooves 221, which separates incoming light into transmitting light (0thorder light) and diffracted light (1st order light) which are focused bylenses 223 and 224, respectively, onto respective light-receivingdevices 225 and 226 to carry out detection. However, also in this case,the diffraction angle of the hologram 222 is large and there must beprovided two lenses 223 and 224 and two light-receiving devices 225 and226 so that the number of parts is not reduced. Furthermore, in FIG. 27,two of the hologram of FIG. 26 are provided to make two light paths,i.e., one for the 0th order light and the other for the 1st order light,extending in the same direction to reduce the installation space.However, also in this case, the number of parts cannot be said to bereduced. It is to be noted that reference should be made to FIG. 11regarding a relationship between the direction of polarization and thelight intensity for the 0th and 1st order light.

In this manner, the prior proposals cannot be said to have successfullyreduced the number of parts in the optomagnetic optical system 210.Moreover, in the prior art approaches, since a reduction of the numberof parts has not been effected extensively for the entire systemincluding both of the optomagnetic optical system 210 and the servocontrol system 209, there has been a problem of incapability to make theentire apparatus compact in size and light in weight.

This aspect of the present invention has been particularly directed toobviate such disadvantages as set forth immediately above and has itsobject to provide an improved optomagnetic optical pick-up compact insize, light in weight and low at cost with a reduction of number ofparts. In accordance with this aspect of the present invention, there isprovided an optomagnetic optical pick-up which comprises a hologramdiffering in groove spacing in an optical path for the light reflectedfrom an optical information recording medium, such as an optical disc(optomagnetic disc). A 2-division light-receiving device is provided inan optical path for the light which has transmitted through and focusedby the hologram. In addition, a 4-division light-receiving device isprovided in an optical path for the diffracted light which has beendiffracted and focused by the hologram. Thus, the light which hastransmitted through the hologram is received by the 2-divisionlight-receiving device to carry out a tracking servo control operation,and the light diffracted by the hologram is received by the 4-divisionlight-receiving device to carry out a focusing servo control operation.Besides, by comparing all of the amounts of light received by the2-division and 4-division light-receiving devices, an optomagneticsignal can be detected. As a result, in accordance with the presentaspect of the present invention, the number of parts can be reducedsignificantly so that there can be provided an optomagnetic opticalpick-up compact in size, light in weight and low at cost.

Now, referring to FIGS. 17 through 20, an optomagnetic optical pick-upfor use in an optomagnetic information recording and reproducingapparatus constructed in accordance with one embodiment of this aspectof the present invention will be described in detail below. Since thebasic structure of the present invention is similar in many respects tothat of the typical prior art shown in FIG. 24 and described above, onlythose portions which relate directly to this aspect of the presentinvention will be shown and explained below while omitting anexplanation regarding the common basic structure. In addition, likenumerals will be used to indicate like elements.

As shown, a hologram 227 is formed with straight grooves 228 withdiffering groove spacing T at one surface thereof and it is disposed inan optical path L₀ for the light which has been reflected from anoptomagnetic disc 207. A 2-division light-receiving device 229 isdisposed in the optical path L₀ for the transmitted light (0th orderlight) which has transmitted through the hologram 227 and for receivingthe light through a focusing lens 230. A 4-division light-receivingdevice 231 is disposed in an optical path L₁ for the diffracted light(1st order light) which has been diffracted by the hologram 227 and forreceiving the diffracted light through a focusing lens 232.

With this structure, the reflected light which has been reflected by theoptomagnetic disc 207 passes through the hologram 227, whereby the lightis separated into two light beams of the 0th order light and the 1storder light. In the first place, the 0th order light is focused by thefocusing lens 230 onto the 2-division light-receiving device 229 tothereby detect a tracking error signal which is used for carrying out atracking servo control operation. On the other hand, the 1st order lightis focused onto the 4-division light-receiving device 231 by thefocusing lens 232 to thereby produce astigmatism, whereby a focusingerror signal is detected to carry out a focusing servo controloperation. With the 0th and 1st order light set to have the lightintensity ratio of 1:1 and by comparing all of the amounts of lightreceived by the 2-division and 4-division light-receiving devices 229and 231, an optomagnetic signal of the optomagnetic disc 207 can bedetected to reproduce the information stored on the disc 207.

As described above, since the reflected light from the optomagnetic disc207 is divided into two light beams by the hologram 227 and the thusdivided light beams are directed toward the 2-division and 4-divisionlight-receiving devices 229 and 231, respectively, a servo detectingoperation, such as a tracking servo detecting operation and a focusingservo detecting operation, can be carried out and also an optomagneticdetecting operation can also be carried out. Thus, the number of partsin these detecting optical systems can be reduced significantly, whichcontributes to make an apparatus compact in size and light in weight.

Next, another embodiment of this aspect of the present invention will bedescribed with reference to FIGS. 21 through 23. This embodiment is amodification of the previously described embodiment and thus likeelements are indicated by like numerals. As shown, a hologram 233 isformed with a plurality of straight grooves 234 spaced apart at an equalinterval at one surface thereof and the other surface of the hologram233 is in contact with the flat surface of the holgram 227. And, thecomposite holgram including the holograms 227 and 233 is disposed in theoptical path L₀ as inclined. The 2-division and 4-divisionlight-receiving devices 229 and 223 are so disposed to receive the lightbeams from the composite hologram through a common focusing lens 230.Thus, the devices 229 and 230 are located on the same side of thefocusing lens 230 in optical paths L₀ and L₁, respectively, which extendgenerally in the same direction.

With this structure, when the light reflected from the optomagnetic disc207 passes through the combined holograms 227 and 233, it is separatedinto two light beams, i.e., the 0th order light and the 1st order light.Then, both of the separated light beams are focused by the focusing lens230 onto the respective light-receiving devices 231 and 229. In thiscase, the 0th order light beam is focused onto the 2-divisionlight-receiving device 229 to be detected as a tracking error signal; onthe other hand, the 1st order light beam is focused onto the 4-divisionlight-receiving device 231 as light having astigmatism to be detected asa focusing error signal. In this manner, by utilizing the light beamsled to the respective 2-division and 4-division light-receiving devices229 and 231, both of a servo detecting operation and an optomagneticsignal detection operation can be carried out similarly with theabove-described embodiment of this aspect of the present invention.Furthermore, since the detecting optical systems are closer together inthe present embodiment, the overall structure of the apparatus can bemade smaller as compared with the previously described embodiment.

Now, a still further aspect of the present invention will be described.This aspect of the present invention relates to improvements in anoptomagnetic optical pick-up for use in an optomagnetic informationrecording and reproducing apparatus. FIG. 33 schematically illustrates atypical prior art optomagnetic optical pick-up. As shown, the laserlight emitted from a semiconductor laser 301 passes through a collimatorlens 302 to be collimated and then passes through a beam shaping prism303 to be shaped to have a circular cross section. Thereafter, the thusshaped light beam passes through a beam splitter 304, a movable mirror305 and an objective lens 306 to be focused onto an optomagnetic disc(not shown).

The reflected light from the optomagnetic disc again passes through theobjective lens 306 and the movable mirror 305, and thereafter it isseparated from the incoming light by the beam splitter to advance towarda detecting system. A part of the light separated by the beam splitter307 passes through an analyzer 308, a mirror 309 and a lens 310 to befocused onto a light-receiving device 311 (e.g., an avalanchephotodiode) to be used for detecting an optomagnetic signal. On theother hand, the remaining portion of the light separated by the beamsplitter 307 is further divided into two light beams by a beam splitter312, one of which passes through a lens 313 to be focused onto alight-receiving device 314 (e.g., pole type 2-division photodiode) to beused for detecting a tracking error signal and a pit signal and theother of which passes through a lens 315, a cylindrical lens 316, amirror 317 and a knife edge 318 to be focused onto a light-receivingdevice 319 (e.g., 2-division photodiode) to be used for detecting afocusing error signal based on a knife edge method.

In such an optomagnetic optical pick-up, since the number of parts ofoptical elements is extremely large as compared with a common opticalpick-up for use in an optical pick-up, only the movable mirror 305 andthe objective lens 306 are provided as an access movable portion 320 toprovide a high-speed access operation to thereby separate the remainingportion from this movable portion 320. However, although an extremelyrigorous driving accuracy is required for the access movable portion320, the actual accuracy is not sufficient so that there occurs faultyreading and malfunctioning.

On the other hand, in order to simplify the optical system, there hasbeen proposed an optomagnetic optical pickup utilizing a polirizinghologram optical element as shown in FIG. 34. In this structure, laserlight emitted from a semiconductor laser 321 passes through a collimatorlens 322, a beam shaping prism 323 and a mirror/focusing lens 324 to befocused onto an optomagnetic disc. The light reflected from theoptomagnetic disc agains passes through the mirror/focusing lens 324 andthe beam shaping prism 323, and, then, through a half wavelength plate325 and a lens 326 to be incident upon a polarizing hologram opticalelement 327 at a Bragg angle so that the light is separated into adiffracted light beam and a transmitted light beam. The diffracted lightbeam is spatially divided into four light beams by the polarizinghologram optical element 327 and received by a light-receiving device328 to be used in the detection of tracking and focusing errors. On theother hand, the transmitted light passes through a birefringent wedge329 to be received by a light-receiving device 230 for use in detectionof the information recorded on the optical disc. The polarizing holgramoptical element 327 is an element for diffracting only the TE polarizedlight (i.e., polarized light in parallel with the grating grooves).Since a signal light component due to the Kerr rotation is the TMpolarized light perpendicular thereto, an excellent signal can bereproduced without a loss in the information detecting system.

Using such polarizing hologram optical element 327, the assembling andadjusting times may be shortened; however, since the light beamseparating angle by the polarizing hologram optical element 327 islarge, it is difficult to make the entire structure smaller to theextent to have this polarizing holgram optical element 327 mounted onthe access movable portion.

This aspect of the present invention has been addressed to obviate theparticular disadvantages set forth above and has an object to provide animproved optomagnetic optical pick-up light in weight, compact in size,easy to manufacture and adjust and fast in operation. Therefore, inaccordance with this aspect of the present invention, there is providedan optomagnetic optical pick up for use in an optomagnetic informationrecording and reproducing apparatus which comprises a plurality ofhologram diffraction grating substrates having different functions and atransparent support having a plurality of mounting surfaces to which theplurality of hologram diffraction grating substrates are attached to bepositioned in a predetermined relative arrangement.

Referring now to FIGS. 29 through 33, an optomagnetic optical pick-upconstructed in accordance with one embodiment of this aspect of thepresent invention will be described in detail below. In a manner similarto that described with reference to FIG. 33, laser light emitted from asemiconductor laser 331 is collimated by a collimator lens 332 and thenshaped to have a circular cross section by a beam shaping prism 333.Then, the light beam thus shaped passes through a beam splitter 334 anda movable mirror 335 and then it is focused onto an optomagnetic disc(not shown) by an objective lens 336. The light reflected from theoptomagnetic disc again passes through the objective lens 336 and themovable mirror 335, and, thereafter, the reflected light beam isseparated from the incoming light beam by the beam splitter 334 to bedirected toward a detecting system through a focusing lens 337.

In this detecting system, there are provided light-receiving elementssuch as an optomagnetic signal detecting light-receiving device 338having a 2-division photodiode structure as shown in FIG. 30a, atracking error detecting light-receiving device 339 having a 2-divisionphotodiode structure as shown in FIG. 30b and a focusing error detectinglight-receiving device 340 having a 4-division photodiode structure asshown in FIG. 30c. It is to be noted that directions indicated by arrowsa and b shown in FIG. 29 correspond to directions indicated by arrows aand b in FIGS. 30a and 30b, respectively, and they indicate thedirection of division in each of the light-receiving devices 338 and 339in FIG. 29.

In the present embodiment, in order to guide the light passing throughthe focusing lens 337 to these light-receiving devices 338, 339 and 340,there are provided three hologram diffraction grating substrates 341,342 and 343 having differing functions. There is also provided agenerally hexagonally shaped transparent support 344 formed with threemounting surfaces 344a, 344b and 344c for mounting thereon these threehologram diffraction grating substrates 341, 342 and 343 so as toarrange them in a predetermined relative positional arrangement. Thetransparent support 344 is comprised of a material which has anappropriate refractive index and which allows a light flux to transmittherethrough, and its outer shape is determined by a relative positionalrelationship of the plurality of hologram diffraction grating substrates341, 342 and 343 to be provided. That is, if the shape and the index ofrefraction of the transparent support 344 are accurate, there can beobtained a detecting optical element capable of carrying out a lightflux separating function without adjustments simply by attaching threehologram diffraction grating substrates 341, 342 and 343 to therespective mounting surfaces 344a, 344b and 344c.

A hologram diffraction grating substrate 341 serves to separate a lightflux incident from above through the focusing lens 337 into ±1st orderdiffraction light fluxes in the left and right directions as indicatedby the arrows c, and thus it is comprised of a relatively coarse (i.e.,its pitch is larger than the wavelength of the laser light) straightgratings. With the hologram diffraction grating substrate 341 present atthe mounting surface 344a of the transparent support 314, the light fromthe focusing lens 337 is divided into two light beams in the left andright directions and the thus divided two light beams travel within thetransparent support 344 toward the mounting surfaces 344b and 344c.

The hologram diffraction grating 342 attached to the mounting surface344b serves to further divide the incoming light beam which has beendivided by the hologram diffraction grating substrate 341 into two lightbeams in a direction vertical to the plane of the drawing, i.e., towarda pair of light-receiving devices 339 and 340 spaced apart from eachother in the direction perpendicular to the plane of the drawing. And,the hologram diffraction grating 342 is comprised, for example, of aplurality of inclined straight gratings 342a and 342b as shown in FIG.31b. More specifically, the region 342a which corresponds to thetracking error detecting light-receiving device 339 is comprised of aplurality of straight gratings which are spaced apart at a predeterminedpitch; on the other hand, the region 342b which corresponds to thefocusing error detecting light-receiving device 340 using an astigmaticmethod is comprised of a plurality of gratings which are spaced apartfrom each other randomly to thereby allow to produce an appropriateastigmatic deviation.

The hologram diffraction grating substrate 343 attached to the mountingsurface 344c serves to separate by polarization the remaining lightwhich has been separated by the hologram diffraction grating substrate341 and guide it toward the 2-division region of a light-receivingdevice 338, and, in particular, it is structured to have a relativelydense (i.e., its pitch is smaller than the wavelength of the laser)linear diffraction gratings on the front and rear surfaces as shown inFIG. 31c so that it has a polarization separation function as in apolarizing beam splitter. In accordance with this embodiment, anoptomagnetic optical pick-up simple in structure, compact in size, lightin weight and easy in adjustment can be obtained simply by attachingthree hologram diffraction grating substrates 341, 342 and 343 to thetransparent substrate 344.

FIG. 32 illustrates an optomagnetic optical pick-up constructed inaccordance with another embodiment of this aspect of the presentinvention. As shown, this embodiment uses a semiconductor laser 351which does not require beam shaping instead of the semiconductor laser331 in the previous embodiment, and, thus, the present embodiment can besimpler in structure. The semiconductor laser 351 is disposed at thecenter among the light-receiving devices 338, 339 and 340. The lightemitted from the semiconductor laser 351 is collimated by a collimatorlens 332 and the thus collimated laser beam passes through thetransparent support 344 and the hologram diffraction grating substrate341. Then, the light is focused onto an optomagnetic disc by anobjective lens 336 straight. In the present embodiment, a beam shapingsystem can be omitted and the optical system as a whole can be madecompact in size and light in weight. Thus, even if an optical system asa whole without separating the access movable portion were driven tomove, a sufficient high-speed access operation could be obtained.Besides, in the present embodiment, since the light-receiving section(i.e., light-receiving devices 338, 339 and 340) and the light-emittingsection (i.e., semiconductor laser 351) are located on the same plane,all of these elements can be disposed on the same substrate with highpositional accuracy and thus requirements for adjustments in positioncan be reduced significantly.

While the above provides a full and complete disclosure of the preferredembodiments of the present invention, various modifications, alternateconstructions and equivalents may be employed without departing from thetrue spirit and scope of the invention. Therefore, the above descriptionand illustration should not be construed as limiting the scope of theinvention, which is defined by the appended claims.

What is claimed is:
 1. In an optical recording and reproducing apparatusfor recording information on an optical information recording medium byapplying light emitted from a semiconductor laser and reproducinginformation recorded on said medium by detecting an optomagnetic signalfrom said medium while carrying out focusing and tracking servo controloperations using light reflected from said medium, an optomagneticpick-up comprising:a hologram disposed in an optical path for lightreflected from said medium and having a plurality of grooves arrangedvarying in spacing; a 2-division light-receiving device disposed in anoptical path for receiving a first light beam which has been transmittedthrough and focused by said hologram; and a 4-division light-receivingelement disposed in an optical path for receiving a second light beamwhich has been diffracted and focused by said hologram.
 2. The pickuprecited in claim 1, wherein said hologram separates the light reflectedfrom said medium utilizing the direction of polarization of saidreflected light.
 3. The pickup recited in claim 1, further including asecond hologram disposed in the optical paths of said first and secondlight beams.
 4. The pickup recited in claim 3, including a singlefocusing lens disposed in the optical paths of said first and secondlight beams to focus said first and second light beams on said2-division device and 4-division light receiving element, respectively.5. The pickup recited in claim 3, in which said second hologramcomprises grooves which are spaced apart at equal intervals.
 6. Thepickup recited in claim 1, in which said grooves are spaced apart atgradually increasing distances.
 7. The pickup recited in claim 1, inwhich each of said first light beam and said second light beam isnon-divergent.
 8. A pick-up for use in an optomagnetic informationrecording and reproducing apparatus for recording information on anoptomagnetic information recording medium by applying light emitted froma semiconductor laser to said medium and reproducing informationrecorded on said medium by detecting an optomagnetic signal from saidmedium while carrying out focusing and tracking servo control operationsusing light reflected from said medium, said pick-up comprising:ahologram disposed in an optical path for light reflected from saidmedium and having a plurality of grooves spaced apart at varyingdistances from each other, said hologram utilizing the direction ofpolarization of said reflected light for transmitting and focusing saidreflected light to thereby generate a first light beam and fordiffracting and focusing said reflected light to thereby generate asecond light beam; a 2-division light-receiving device disposed in anoptical path for receiving said first light beam; and a 4-divisionlight-receiving device disposed in an optical path for receiving saidsecond light beam.
 9. A pickup as recited in claim 8, further comprisingmeans for detecting the difference between the transmitted light of saidfirst light beam detected by said 2-division light-receiving device anddiffracted light of said second light beam detected by said 4-divisionlight receiving device.
 10. A pickup as recited in claim 8, including asecond hologram disposed in the optical paths of said first and secondlight beams.
 11. A pickup as recited in claim 10, including a singlefocusing lens disposed in the optical paths of said first and secondlight beams to focus said first and second light beams on said2-division and 4-division device, respectively.
 12. A pickup as recitedin claim 10, in which said second hologram comprises grooves which arespaced apart at equal intervals.
 13. A pickup as recited in claim 8, inwhich said grooves are spaced apart at gradually increasing distances.14. A pickup as recited in claim 8, in which each of said first lightbeam and said second light beam is non-divergent.
 15. A method forrecording on and reproducing information from an optomagneticinformation recording medium by applying light emitted from asemiconductor laser and reproducing information recorded on said mediumby using a pickup to detect an optomagnetic signal from said mediumwhile carrying out focusing and tracking servo control operations usinglight reflected from said medium, comprising:disposing, in an opticalpath for light reflected from said medium, a hologram having a pluralityof grooves spaced apart at varying distances and utilizing the directionof polarization of said reflected light, to thereby generate a firstlight beam which has been transmitted and focused by the hologram and asecond light beam which has been diffracted and focused by the hologram;receiving said first light beam at a 2-division light-receiving devicefor generating therefrom a tracking error signal; and receiving saidsecond light beam at a 4-division light-receiving device for generatingtherefrom a focusing error signal.
 16. The method recited in claim 15,wherein the difference between said first light beam detected by said2-division light-receiving device and said second light beam detected bysaid 4-division light beam is representative of said optomagneticsignal.
 17. The method recited in claim 15, further including disposinga second hologram in the optical paths of said first and second lightbeams.
 18. The method recited in claim 15, including focusing said firstand second light beams on said 2-division device and 4-division device,respectively, by a single focusing lens disposed between the secondhologram and said devices.